This invention relates to in-situ vitrification (ISV), and particularly to means for sensing the progress of an ISV process.
In-situ vitrification (ISV) has been proposed as a process for in-place stabilization of a contaminated soil formation. The ISV process is carried out by placing four electrodes in the soil formation at the corners of a square. Tests indicate that electrode separations of at least 5.5 m may be used, allowing treatment of a site having a surface area of over 30 m.sup.2. A conductive mixture of flaked graphite and glass frit is placed in an X pattern between the electrodes, in shallow trenches on the soil surface. Voltages are applied to the electrodes, and the graphite mixture conducts current between the electrodes. The graphite mixture is quickly heated to soil-melting temperatures (1,100 degrees to 1,600 degrees C.). As the surrounding soil melts, it becomes electrically-conductive and the affected soil volume grows downward and outward until the desired soil volume has been completely vitrified and the waste material is thereby encapsulated in the vitreous mass.
It is desirable that the depth to which the ISV process has progressed be monitored, in order to determine that the process is proceeding as expected. It is also desirable to be able to ensure that the soil has been vitrified to the desired depth before the ISV operation is stopped. The soil temperature may reach 2,000.degree. C., and therefore methods for determining depth are restricted to those that do not intrude into the molten zone. At such high temperatures, the soil emits radiant light.